Films comprised of polyesters in the stated thickness range are well known. A disadvantage of these polyester films, however, is their susceptibility to hydrolysis, in particular at temperatures above the glass transition temperature of the respective polyester. Susceptibility to hydrolysis here is the property of the polyester of becoming hydrolytically degraded under moist conditions, this being discernible by way of example from a reduction in the IV or SV value (viscosity). This is particularly a limiting factor for the use of polyester films in applications with relatively great exposure to high temperatures, for example in film capacitors, cable sheathing, ribbon cables, and engine-protection films, but also in long-term applications, for example in glazing and outdoor applications.
Susceptibility to hydrolysis is particularly pronounced with aliphatic polyesters, but also with aromatic polyesters, such as PBT and PET. If the susceptibility of PET to hydrolysis becomes too great for the application, it becomes necessary to resort to PEN, which is somewhat more hydrolysis-resistant, or even to other polymers, e.g. polyetherimides or polyimides. However, these are markedly more expensive than PET and therefore are frequently unsuitable for economic reasons.
For this reason, there have been previous proposals for improving the hydrolysis resistance of polyester films via incorporation of hydrolysis stabilizers.
Polyesters relatively resistant to hydrolysis, obtained via use of carbodiimides, are known (U.S. Pat. No. 5,885,709, EP-A-0 838 500, CH-A-621 135), as also are films and fibers produced therefrom. Films produced from these polymers tend, however, both during production and in later use, to evolve gases of isocyanates and of other by- and degradation products which are hazardous to health or irritant to mucous membrane. This problem is much more widespread with sheet-like structures, such as films, with large surface area, than with injection moldings or the like.
Hydrolysis stabilizers which have epoxy groups also give hydrolysis stabilization and are described by way of example in EP-A-0 292 251 A2 or U.S. Pat. No. 3,657,191. However, these compounds are based on the production of oxirane rings by means of epichlorohydrin and have a tendency inter alia caused by their terminal epoxy groups toward cleavage of low-molecular-weight toxic compounds on heating, the problems attendant on the use of these substances therefore being similar to those attendant on the use of carbodiimides. Furthermore, their incorporation into the polyester matrix is inadequate, and this leads to long reaction times and to high, undesired haze in the case of oriented polyester films.
Another disadvantage of known hydrolysis stabilizers, such as carbodiimide and other substances such as those described in EP-A-0 292 251 is that they sometimes lead to marked increases in molecular weight (viscosity rise) in the polymer during extrusion, thus making the extrusion process unstable and difficult to control.
DE 103 49 168 A1 describes mixtures comprised of epoxidized alkyl esters of fatty acid and of epoxidized fatty acid glycerides as hydrolysis stabilizers inter alia for industrial oils, vegetable esters, polyurethanes, and polyesters, the preferred embodiment of these comprising, as third component of the mixture, up to 30% by weight of a carbodiimide. There are no indications of appropriate incorporation of these hydrolysis stabilizers into a polyester film. However, when these additives are used in particular in outer layers of the film (in the case of a single-layer film this layer being the outer layer), the result is impairment of metallizability/printability, and odor problems can occur via evaporation of the stabilizer.